1. Field of the Invention
The present invention relates to a method of eliminating or reducing the detrimental effects resulting from deposition of organic contaminants on surfaces in paper process systems. More specifically the invention is for the use of synergistic combinations of hydrophobically modified hydroxyethylcellulose and cationic polymers to inhibit deposition of organic contaminants onto surfaces of papermaking equipment.
2. Description of Related Art
Paper production is a process during which cellulosic fibers (pulp) isolated from wood or recycled paper are suspended in water (pulp slurry) and directed to the wire section of a papermachine where water is drained from the pulp suspension to create a paper web. During subsequent processing of the paper web on the paper machine, the water content of the paper web is reduced as the paper sheet is formed and dried. While paper is produced, several different types of surfaces on the machine are contacted by the pulp slurry, the paper web, the paper sheet, as well as the water used to transport the pulp slurry. Contact with surfaces of the paper machine or components thereof can result in some contaminating organic materials in the process water stream adhering to or depositing onto the surfaces. Within pulp production or processing facilities, exposed surfaces include screen rooms and deckers. Surfaces on parts of papermachines can be made of metal, granite, ceramic, mylar, polyester, plastic, and other synthetic materials. Such surfaces include machine wires, felts, foils, uhle boxes, headbox components, press rolls, fabric carrier rolls, calendar rolls, Doctor blades, and dryer cans and fabrics. Proper operation of the paper machine requires that surfaces be reasonable free of deposits of contaminating materials. The terms “papermaking system” and “paper process system” are meant to include all processes, including pulp production, that are part of paper production.
Contaminating materials in a paper process system that deposit onto surfaces of papermaking equipment are generally referred to as pitch or stickies. In the strictest sense, pitch is a term used to refer to any organic matter originating from the extracts of wood including fatty acids and esters, resin acids, and sterols. Pitch that is not removed in the pulp mill with washers and/or cleaners can deposit on papermaking equipment surfaces. Pitch deposits may contain other materials such as defoamers, sizing agents, coatings, inorganic components (i.e., calcium carbonate, silica, clay, magnesium, and/or titanium).
If the source of the cellulosic fiber used to produce paper is recycled paper, deposits of contaminating materials may include materials referred to as stickies. Cellulosic fiber from recycled paper can include significant quantities of thermoplastic impurities that come from self-adhesive envelopes, latex in coatings, hot melts, polyethylene films, pressure sensitive adhesives, and waxes. These impurities make up stickies. Depending on the source of the cellulosic fiber (stock), stickies and pitch can form in the same deposit. A stickies deposit may include components of pitch as well as chemicals used in papermaking. The common approach to controlling stickies is to use mechanical and chemical programs. Chemical programs are designed to control contaminants that are not removed from the system during the flotation stage of the de-inking process. Chemicals used to control contaminants include talc, polymers, dispersants, and surfactants.
Pitch or stickies deposition is detrimental to efficient production of paper and the operation of paper mills. Pitch and/or stickies deposit, on surfaces exposed to the pulp slurry or process water removed during sheet formation causing operational problems in the systems. For example, modern paper machines have a variety of process monitors as integral components of the papermachine. Pitch deposits on process monitors can render these components useless. Deposits of pitch on screens can reduce throughput and cause disruptions in the operation of the paper mill. Stickies and pitch can also adversely affect the quality of the finished paper sheet. Parts of deposits can become dislodged from a contaminated surface, become integrated into the paper web, and form spots or other defects in the sheet. Deposits of stickies or pitch on rollers can cause defects on the surface of the paper.
Low concentrations of fine particles of pitch or stickies that remain well dispersed do not create a deposition problem. However, there is a tendency for the hydrophobic particles to agglomerate at the air-water interface to form larger aggregates of material, which then deposit on paper making equipment. The degree to which pitch or stickies deposit on a surface is influenced by characteristics of the pitch or stickies and of the paper process system. Characteristics or factors of the pitch or stickies include the composition and stability of the particles, size of the particles, the tendency of the particles to deposit and the amount of pitch or stickies in the systems. Characteristics of the paper processing system that influence or help determine the degree of pitch deposition includes nature of the surface, including affinity of the surface for pitch, temperature, pH, source of fiber, and degree of recycling of water within the paper mill.
Pitch and stickies control programs are system-specific because of the uniqueness of each papermill. A typical pitch control strategy can begin with the addition of nonionic or anionic surfactants that stabilize the colloidal form of the pitch in whitewater. The objective of adding a stabilizing chemical is to preserve the colloidal form of the pitch thereby preventing large agglomerations from forming and depositing on papermachine surfaces. If any pitch colloids form large agglomerations or deposit on surfaces, strongly anionic surfactants (referred to as dispersants) can be used to disperse the pitch. A negative aspect of the use of dispersants is that they can interfere with some functional chemistries such as additives used to retain the colloidal pitch in the paper sheet and sizing.
Rendering pitch and stickies particles to be less prone to deposit is only one aspect of a successful control program. In many papermaking systems, pitch and stickies must be removed from the process stream for paper production to continue. Removing pitch or stickies from paper process system will avoid having concentrations of these contaminants increase to the point that deposition becomes problematic. A common strategy to remove pitch or stickies colloids from a system is to bind the colloids to the paper fibers by feeding certain chemical additives into the papermaking process water that will facilitate the pitch becoming associated with the paper fibers via direct or indirect binding.
The heterogenous chemical composition of pitch and stickies adds complexity and expense to its control. A range of hydrophobic chemicals can be present in pitch and additional hydrophobic chemicals may become associated with pitch during paper production. A common practice to control pitch has been to add alum as part of the chemical pulping process. Soaps of resin acids formed during pulping will associate with alum and these complexes can serve to bind pitch particles to the fiber surface. More recently, highly cationic polymers are added to paper process streams to retain pitch onto the fiber. This is a very important process as it provides a path for the pitch to be continuously removed from the paper process water.
Certain conventional monomeric organic and inorganic chemicals have been shown to be effective in dispersing pitch particles thereby preventing deposition on surfaces of papermaking equipment. Compounds such as sodium polyacrylate and arylsulfonic acid condensates have been shown to be useful for preventing pitch.
Several different classes of chemicals have been reported to be effective in controlling deposition of pitch and stickies. These include surfactants, anionic polymers and copolymers composed of anionic monomers and hydrophobic monomers, talc, alum, bentonite, diatomaceous silica, starch, animal glue, gelatin and some other proteins, and some highly cationic polymers. Other substances include polymeric N-vinyl lactam, xylene sulfonic acid-formaldehyde condensates, and salts thereof, water soluble dicyandiamide-formaldehyde condensates, and certain water-soluble non-surface-active cationic quaternary ammonium salts. Nonylphenol ethoxylate compounds have been used to inhibit pitch deposition in papermaking systems.
European Patent Application 599 440 discloses a pitch dispersant composition comprising blends of certain non-ionic surfactants and water-soluble cationic polymers.
European Patent Application EP 0568229A1 discloses that HMHEC (hydrophobically modified hydroxyethyl cellulose) and related molecules are effective in preventing deposition of pitch and stickies. However, this application only provided evidence that HMHEC is effective for preventing deposition.
Results reported by Shetty et al. (Tappi J. 77, 10: 91, 1994) teach how pitch control can be achieved by adding certain cationic polymers to the fiber furnish. For example, poly-DADMAC polymers promoted coalescence of pitch particles, allowing them to be retained in the paper.
The prior art teaches that certain combinations of chemicals can be effective in preventing pitch deposition while not affecting pitch retention. For example, Dreisbach et al. (U.S. Pat. No. 5,074,961) discloses that water soluble cellulose ethers selected from the group consisting of methyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, carboxymethyl methyl cellulose, and methyl hydroxybutyl methyl cellulose are effective in preventing pitch deposition while not adversely affecting sizing, fines retention, or pitch retention. Furthermore, it was disclosed that the cellulose ethers flocculated and retained pitch.
The prior art also teaches that certain chemicals can be used in combination to decrease pitch deposition while increasing pitch retention. Nguyen (U.S. Pat. No. 5,723,021) disclosed that a combination of polyvinyl alcohol, a high molecular weight gelatin, and a cationic polymer gave decreased deposition and increase retention of pitch in a paper process system.